Lawsonite
Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to translucent colorless, white, pink, and bluish to pinkish grey glassy to greasy crystals. Refractive indices are nα = 1.665, nβ = 1.672 – 1.676, and nγ = 1.684 – 1.686. It is typically almost colorless in thin section, but some lawsonite is pleochroic from colorless to pale yellow to pale blue, depending on orientation. The mineral has a Mohs hardness of 7.5 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture. Not to be confused with Larsonite, a fossiliferous jasper mined in Nevada.
| Lawsonite | |
|---|---|
Sample from the type locality in California with two elongated, lustrous and translucent crystals of pastel pink, lawsonite in mica schist. Size: 6.1 cm × 3.2 cm × 2.5 cm (2.4 in × 1.3 in × 1.0 in) | |
| General | |
| Category | Sorosilicate |
| Formula (repeating unit) | CaAl2Si2O7(OH)2·H2O |
| IMA symbol | Lws |
| Strunz classification | 9.BE.05 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Cmcm |
| Unit cell | a = 5.847, b = 8.79, c = 13.128 [Å]; Z = 4 |
| Identification | |
| Color | Colorless, white, pale blue to grayish blue |
| Crystal habit | Commonly prismatic, tabular; also granular, massive |
| Twinning | Common on {101} lamellar |
| Cleavage | Perfect on {100} and {010}, imperfect on {101} |
| Tenacity | Brittle |
| Mohs scale hardness | 7.5 |
| Luster | Vitreous, greasy |
| Streak | White |
| Diaphaneity | Translucent |
| Specific gravity | 3.05–3.12 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.665 nβ = 1.672–1.676 nγ = 1.684–1.686 |
| Birefringence | δ = 0.019–0.021 |
| Pleochroism | Weak; X = blue, pale brownish yellow; Y = deep bluish green, yellowish green; Z = colorless, yellowish |
| 2V angle | Measured: 84°–85° |
| Dispersion | Strong, r > v |
| References | |
Lawsonite is a metamorphic mineral typical of the blueschist facies. It also occurs as a secondary mineral in altered gabbro and diorite. Associate minerals include epidote, titanite, glaucophane, garnet and quartz. It is an uncommon constituent of eclogite. Its scarcity in eclogite that has been exhumed to the Earth's surface does not reflect its abundance at depth in subduction zones but rather the fact that lawsonite is easily replaced by other minerals.
Lawsonite was first described in 1895 for occurrences on Ring Mountain of the Tiburon peninsula, Marin County, California and was named after geologist Andrew Lawson (1861–1952) of the University of California by two of Lawson's graduate students, Charles Palache and Frederick Leslie Ransome.